I have one, actually I think two. One has long arm that was slightly bent in when I hit the deck, but doubtless can be bent back. It would not be costly.
2nd thought: not sure if it's 11S
Let me know what you find, thanks mate.
Do you need 105? I have 6800 ultegra, has scratches but could let it go for £10
I have a a new and unused RD-5800 in black but...it lacks a lower guide pulley
...but I see this chap has exactly what you need:
As the title says shimano 105 11 speed rear mech, black or silver required, let me know what you’ve got laying around.
In London if you prefer not to post it.